1. Field
The present disclosure relates generally to power converters, and, more specifically, the present disclosure relates to sensing the output voltage of a power converter.
2. Related Art
Many electrical devices, such as cell phones, personal digital assistants (PDAs), laptops, and the like, are powered by relatively low-voltage, direct-current (dc) power sources. Since power is typically delivered through a wall outlet as high-voltage, alternating-current (ac) power, a device generally referred to as a switching-power converter is typically used to transform the high-voltage ac power to low-voltage dc power. These converters generally use a controller to switch a power switch between an ON state and an OFF state to control the amount of power transmitted to the output of the converter and delivered to a load.
In certain applications, switching-power converters may include an energy transfer element to separate an input side of the power converter from an output side of the power converter. More specifically, an energy transfer element may be used to provide galvanic isolation that prevents dc current between the input and the output of the power converter. Common examples of energy transfer elements include transformers and coupled inductors, where electrical energy is converted to magnetic energy that is converted back to electrical energy at the output side across an output winding.
One typical way of regulating power, referred to as primary-side regulation, may include obtaining feedback information using a bias winding that is electrically coupled to the input side of the power converter such that the bias winding is also magnetically coupled to the output winding of the energy transfer element. This allows the bias winding to produce a voltage representative of the output voltage of the power converter that is accessible from the input side. In this manner, the switching-power converter may acquire a feedback signal representative of the output voltage without directly sensing the output voltage at the output of the converter. During operation, the power converter may regulate its output voltage by adjusting the frequency and duration of the switching events in response to the feedback from the bias winding. By adjusting the frequency and duration of the switching events, the converter may control the amount of energy transferred from an input of the power supply to the output.
While this method is generally effective during certain transient conditions, primary side sensing may obtain false samples of the feedback signal. Typically, the feedback signal from the bias winding is representative of the output voltage only a portion of the time. Thus, when implementing primary side control, sensing may only be conducted during specific times when the bias winding voltage is representative of the output voltage. A controller may be designed to sense the bias voltage at a very specific time. In certain cases, the controller may sense the bias voltage at an incorrect time due to pre-set sensing signals that may not account for a transient condition and therefore not sample the output voltage waveform at the appropriate time that it is representative of the output voltage. As a result, the power converter may determine energy delivery based on incorrect information. This may cause the output voltage to fall out of regulation.